1. Field of the Invention
The present invention relates to Ce3+ doped silicate based phosphors and white light emitting diodes including Ce3+ doped silicate based phosphors, more particularly, to a Ce3+ doped silicate based phosphor with superior emission efficiency when applied to light emitting diodes and/or active light emission type liquid crystal displays, characterized in that the phosphor is formed by adding cerium oxide (CeO2) as an activating ingredient to a calcium silicate base and heat treating the mixture under specific conditions, as well as its preparation method and white light emitting diodes manufactured using the same.
2. Background
White light emitting diodes (“white LEDs”) are well known as one of future light emitting devices capable of replacing traditional fluorescent or incandescent lighting devices. White LEDs are superior to conventional lighting devices due to their reduced power consumption requirements, higher light emission efficiency, high brightness, extended lifetime of use, and short response time.
White LEDs are generally manufactured by three kinds of methods including: a method for mounting together three different diodes with red, green and blue light emission, respectively; a method for coating all of red, green and blue light emitting phosphors on a near-UV LED; and a method for coating a yellow emitting phosphor on a blue LED.
For the first method for mounting together all of three diodes with red, green and blue light emission, respectively, a light emitting device is formed using three chips and requires alternative semiconductor thin films to manufacture separate LEDs with blue, green and red light emission, and it causes increase of investment cost in a process for manufacturing LEDs and unit price of the product.
The second method for coating three phosphors with red, green and blue light emission, respectively, on a near-UV LED was disclosed in International Patent Publication No. WO 98/039805. This is the most preferable method to generate R-G-B type white light by transmitting UV light to phosphors for three primary colors. However, the prepared LED excessively emits heat and has poor emission efficiency. There is still a requirement for developing novel phosphors with improved light emission efficiency for near-UV light. For instance, products available from NICHIA Co., or Toyoda Gosei Co., Ltd., Japan, also have small output power of only 2 to 3 mW. Since transparent resins for coating near-UV LED chip have not yet been developed, organic resins are mostly used at present to cover the chip. Such organic resins absorb UV light and are deteriorated by UV light, which reduces the lifetime and quality of LEDs comprising these materials.
The third method for preparing white LEDs comprises coating yellow light emitting phosphors on a blue LED, which has been widely and intensively investigated. This method has advantages such as easy production of diodes with simple structures and emission of white light with high brightness. This method has been described in detail in International Patent Publication No. WO 98/05078 owned by NICHIA Co., as well as a by S. Nakamura et al., “The Blue Laser Diode,” Springer-Verlag, p. 216-219 (1997). White LEDs prepared by this method operate as follows: blue light emitted from a LED is first absorbed in a phosphor composed of yttrium-aluminum-garnet (Y3Al5O12:Ce3+, referred to herein as “YAG”), which then emits yellow light to form white light by combination of the blue and yellow spectral regions. However, YAG-based light emitting phosphors have problems including a relatively weak emission strength in the red (long wavelength) spectral region due to characteristics of light emission wavelength, and color-dependent sensitivity to temperature. These problems may be an obstacle to achieving adequate color rendering features, and make the phosphor unsuitable for use in general lighting and/or light source for backlight units of color liquid crystal display panels.
With regard to the above description, Korea Research Institute of Chemical Technology reported Eu2+-doped strontium silicate phosphor (Sr3SiO5:Eu2+) in Korean Patent Laid-Open No. 2004-0085039. However, since the phosphor has a center wavelength of about 570 nm with a narrow emission bandwidth and emits orange light rather than yellow light, there is a problem of not being able to emit white light with high color temperature of more than 6500 K when the phosphor is combined with a blue LED. Alternatively, Korea Research Institute of Chemical Technology reported Eu2+-doped Sr2SiO4 phosphor in Korean Patent Laid-Open No. 2004-0069547, but here also the phosphor exhibited poor color rendering features due to a narrow emission bandwidth.